Spare a little pity for Jerry Brown. The California governor has been standing up admirably to Donald Trump on many issues, but especially on climate change—even threatening to launch scientific satellites to replace the ones that Washington wants to ground. This week, he’s in Bonn, Germany, at the global climate talks, spearheading the drive to show that America’s states and cities have not forsaken the promises made last year in Paris. On Saturday, barely a minute into his big prime-time talk, Brown was rewarded for his pains with booing. He was visibly startled when demonstrators interrupted his speech and began chanting, “Keep it in the ground!”

Pity him, then, but not too much. For one thing, Brown responded to the challenge Trumpishly—“let’s put you in the ground,” he told the protesters, who were led by indigenous and climate-justice activists. And, for another, they were absolutely right; their slogans illustrated the contradiction at the heart of the planet’s climate policy, one that Brown, if he wanted to, could play a key role in solving.

There are two halves to the climate dilemma: demand and supply. We use too much coal and gas and oil, and we’ve begun to address that through the rapid adoption of renewable energy, the spread of conservation measures, and ideas such as a price on carbon. Brown’s California has been a leader in much of this work. But we also produce too much fossil fuel, and that endless production makes it harder to drive down demand. In fact, it will make it impossible to meet even the modest goals of the Paris accords. A remarkable study, published last year by Oil Change International, found that the world’s developed oil and gas fields—the ones we’re already pumping—contain enough carbon to carry us past the 1.5-degree-Celsius temperature increase agreed to in Paris. (Add coal to the mix and we go way past two degrees, without ever discovering another seam or field.) That’s why campaigners from around the world, meeting in Lofoten, Norway, this summer, signed a declaration calling on governments to begin the “managed decline” of the world’s fossil-fuel-production zones.

Five hundred N.G.O.s—including 350.org, which I helped found—have signed that declaration, but not many political leaders. In fact, heads of governments tend to fall into one of two camps. The first, populated largely by Trump and his followers, sees climate change as nonsense and aims to increase both supply and demand. The other, which includes everyone from Barack Obama to Canada’s Justin Trudeau to Brown, offers inspiring rhetoric on fighting global warming but refuses to rein in fossil-fuel exploration and development…